Bacteriophages are the most abundant biological entity in the biosphere. They facilitate the evolution of bacterial pathogenicity by imposing selection for resistance to infection and by horizontal gene transfer of host genes to new bacteria. More specifically, phages often carry toxins and virulence factors that convert benign bacteria into human pathogens, facilitating the spread of bacterial infections. Most phages utilize elaborate tail machines to translocate their viral DNA and proteins, across bacterial membranes, into a host cell. In addition, these highly sophisticated molecular machines are responsible for host-cell recognition, attachment, and cell wall penetration. However, initial adsorption and genome ejection remain the least understood aspects of any phage life cycle. The central hypothesis is that the tail machine undergoes a cascade of coordinated conformational changes to efficiently infect a host bacterium. The objective of this application is to document these conformational rearrangements by determining intermediate structures of Podoviridae T7, Myoviridae T4, and Siphoviridae ? during infection by combining high throughput cryo-electron tomography (cryo- ET) with molecular genetics of both phage and host. Comparative structural analysis of these three morphotypes, together with a wealth of biochemical and structural information, will provide new insights into the mechanistic pathways of phage infection at a molecular level.